The Collaborative International Dictionary of English v.0.48:

Unstring \Un*string"\, v. t. [1st pref. un- + string.]
   [1913 Webster]
   1. To deprive of a string or strings; also, to take from a
      string; as, to unstring beads.
      [1913 Webster]

   2. To loosen the string or strings of; as, to unstring a harp
      or a bow.
      [1913 Webster]

   3. To relax the tension of; to loosen. "His garland they
      unstring." --Dryden. Used also figuratively; as, his
      nerves were unstrung by fear.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unstring
    v 1: remove the strings from; "unstring my guitar" [ant:
         string]